1. Field of the Invention
The present invention relates to a nonlinear optical device used for converting optical wavelength utilizing nonlinear optical effect and its material.
2. Description of the Prior Art
In recent years, there have been proposed various devices using the large nonlinear optical effect and the high speed response of organic nonlinear optical materials.
Especially, an optical wavelength converter utilizing second-order nonlinear optical property has been a focus of study as light sources, etc., for an optical disc driver, a laser beam printer, and so on, because such a converter can be utilized for making the wavelength of a semiconductor laser light shorter.
A nonlinear optical waveguide device for converting optical wavelength has high electric field strength in the waveguide, which enables the propagation of light over a long distance, thereby promising high efficiency of converting a fundamental wave to higher harmonics.
When optical wavelengths are converted in a waveguide device, as a phase matching process, there have been proposed phase matching between modes (see e.g., Optics Communications 47, 347-350 (1983); Optics Communications, 59, 299-303, (1986)), and a process utilizing Cherenkov radiation (see e.g., Japanese Journal of Applied Physics, 28, 9, 1622-1628 (1989); Proceedings of SPIE, 682, 187-190 (1986)).
When the phase matching between modes is utilized, the conversion efficiency largely changes according to differences from the set values of the waveguide thickness and temperature, thereby making it difficult to stably manufacture practical devices.
On the other hand, when Cherenkov radiation is utilized, the conversion efficiency does not largely vary according to changes in temperature and the waveguide thickness, and therefore a device with high conversion efficiency showing promise for the practical use thereof.
FIG. 4 is a graph showing the relationship between the second-order polarizability .beta. and the wavelength at the absorption edge of the molecules of well known organic nonlinear optical materials. In the graph, POM is 3-methyl-4-nitropyridine-1-oxide, m-NA is m-nitroaniline, MNA is 2-methyl-4-nitroaniline, and DANS is 4-dimethylamine-4'-nitrostilbene (since DANS forms centrally symmetric crystals, when used as a crystal thereof, it exhibits no wavelength conversion property).
FIG. 4 indicates that, with an increase in the conjugated system and wavelength at the absorption edge of the molecules, the value of .beta. markedly increases.
Generally, the efficiency of converting optical wavelength is proportional to the square of the value of .beta.. This indicates that the use of materials having longer wavelength at the absorption edge is more advantageous for raising the conversion efficiency.
Actually, the wavelength of the fundamental waves used in nonlinear optical devices lies in the range of 0.7 .mu.m to 1 .mu.m, i.e., the wavelength of a YAG laser light or a semiconductor laser light.
Considering that the wavelength of the second harmonic wave is half of that of a fundamental wave, nonlinear optical materials are required to transmit the second harmonic light so as not to absorb it. Therefore, in the prior art, it has been considered that materials having a wavelength at their absorption edge of about 0.5 .mu.m or less are needed for converting the wavelength of a YAG laser light, and materials having a wavlength at their absorption edge of about 0.45 to about 0.35 .mu.m or less are needed for converting the wavelength of a semiconductor laser light. (see e.g., "Nonlinear Optical Properties of Organic Molecules and Crystals Vol.1" (1987), D. S. CHEMLA et al., Academic Press).
Various materials have been examined which have a wavelength at their absorption edge of 0.45 to 0.35 .mu.m and have a large .beta. value, but materials having a large .beta. value applicable for practical use have not yet been obtained.
Therefore, devices using such conventional nonlinear optical materials do not have sufficient conversion efficiency, which leads to a drawback in that the devices have low strength of light when used as light sources for an optical disc driver, a laser beam printer and so on.